Based on the different geometric design and calculation of involute internal gear pair, the calculation methods and applicability of involute internal gear pair with few teeth difference were presented when using different processing methods, including hobbing, gear shaper and wire cut EDM, and different top gaps. The influenced factors such as modification coefficient, engaging angle, contact ratio and the interference of tooth profiles were investigated via different calculation methods of addendum circle diameter. To meet the constraints of contact ratio and tooth profile interference, modification coefficient and the minimum angle of engagement were obtained by using Newton’s iterative method through different calculation methods of addendum circle diameter. The calculation results indicated that under the restrictive conditions of contact ratio and tooth profile interference, regardless of hobbing or gear shaping process, angle of engagement and modification coefficient of the top gap of mixed mode of involute internal gear pair with few teeth difference were smaller when obtaining the minimum engaging angle.